Silicon nitride has been a material of interest for many years due to its high temperature strength, creep resistance and low thermal expansion, as well as its extremely efficient resistance to corrosion and its ability to make critically engineered parts.
Generally, the practice of nitriding silicon-containing material has been investigated for many years, and has resulted in a process which takes a very long time, so long in fact that the process is not commercially feasible. It would be advantageous to practice a method for nitriding the silicon-containing material to produce silicon nitride in a manner which takes less time than current methods. It would also be advantageous to have a method of enhancing the nitridation of silicon so that a more complete conversion to silicon nitride may be achieved.
Generally, it has been the practice to form silicon nitride parts by "reaction bonding" or by "hot pressing" or by a pressureless sintering process. Reaction bonding comprises compacting silicon powder of commonly less than 400 mesh into the part commonly at ambient temperature and then exposing the part to molecular nitrogen at about 1400.degree. C. for a period of time sufficient to convert the silicon to silicon nitride such as disclosed in U.S. Pat. No. 4,235,857, the disclosure of which is incorporated herein by reference. Such is also reviewed by A.J. Moulson in an article titled "Review Reaction-Bonded Silicon Nitride: Its Formation and Properties," Journal-Materials Science, 14, (1979) 1017-1051 and by Mangels in an article titled "Effect of Rate-Controlled Nitriding and Nitriding Atmospheres on the Formation of Reaction-Bonded Si.sub.3 N.sub.4 ", Ceramic Bulletin, Volume 60, No. 6 (1981), 613 in which he also describes benefits derived by using a nitriding gas mixture of nitrogen with minor amounts of hydrogen and helium. The use of a combined nitrogen-hydrogen nitriding gas in the reaction bonding of Si.sub.3 N.sub.4 is described by Shaw and Zeleznik in an article titled "Thermodynamics of Silicon Nitridation: Effect of Hydrogen", communications of the American Ceramic Society, November 1982, C180-C181 and the effect of temperature and nitrogen pressure on the kinetics of silicon nitridation along with the need for an activating agent such as iron is described by Atkinson, Moulson and Roberts in an article titled "Nitridation of High-Purity Silicon", Journal American Ceramic Society, Volume 59, No. 7-8, 285-289.
It has been the practice to nitride silicon powder by heating for long periods of time. An example of such is disclosed in U.S. Pat. No. 3,819,786, where a blend of silicon nitride powder and a binder mixture is heated in a stream of nitrogen from ambient to 1000.degree. C. at 50.degree. C./hr and then held under static nitrogen for 20 hours at 1350.degree. C. and 10 hours at 1450.degree. C. with a total nitriding time of more than thirty hours.
An example of a compound heating schedule for nitriding a mixture of silicon and silicon carbide powder is disclosed in U.S. Pat. No. 3,222,438, the disclosure of which is incorporated herein by reference, where the mixture is first compacted into a green compact and then heated in an atmosphere of nitrogen at a temperature of 1250.degree. C. for 16 hours and then at 1450.degree. C. for 3-4 hours where the first stage heating is conducted to pre-sinter the compound so that it doesn't melt at the 1450.degree. C. temperature since the melting point of silicon is about 1420.degree. C.
Reaction bonded silicon nitride is commonly prepared by reacting and nitriding the silicon (either as a powder or as a formed article) with nitrogen by exposing the silicon to a nitrogen-containing atmosphere at temperatures of 1100.degree. C. to about 1420.degree. C. for times sufficient to produce the silicon nitride. It is not uncommon for the nitriding time in prior art methods to be about 100-200 hours. It is normal for a small amount of nitriding aid (e.g., iron oxide or nickel oxide) to be initially mixed with the silicon powder to enhance the nitridation of the silicon during the nitriding step. U.S. Pat. No. 3,206,318 to Yamauchi et al. teaches a method of nitriding metallic silicon which lowers the ill effects of the oxidation of silicon nitride, in which the nitriding catalyst is (a) at least one primary substance selected from the group consisting of metallic vanadium, the inorganic compounds thereof, and mixtures thereof; or (b) that comprising (a) in which has been incorporated at least one secondary substance, selected from the group consisting of metallic cobalt, manganese, chromium, copper, nickel, iron, barium, and calcium and the inorganic compounds thereof. Yamauchi, et al. also teach a refractory article in which granular refractory material, such as alumina, is bonded with silicon nitride. The patent furthermore teaches that the oxides of the metals, Cu, Co, Ni, Cr, Mn and V, may likewise be used and that the quantity of these oxides is suitably 0.1-2 moles in terms of the metallic element to 100 moles of the silicon.
U.S. Pat. No. 4,235,857, METHOD OF NITRIDING SILICON, to Mangels teaches that silicon can be nitrided using a nitriding cycle over the temperature range of 900.degree. C. to 1420.degree. C. in an atmosphere consisting of a mixture of nitrogen, hydrogen and helium. However, the chemical composition of the nitriding gas is constantly changing due to the consumption of nitrogen during the nitridation of the silicon article, with the chemical activity of the nitrogen decreasing (partial pressure of nitrogen in the furnace decreases) as the temperature increases. The examples cited by Mangles have nitriding times of from 130 to 175 hours.
U.S. Pat. No. 4,351,787 to Martinengo et al. teaches that sintered silicon nitride articles can be prepared by forming a silicon powder mixture containing one or more sintering additives into a compact, the additives being present in the powder in an amount such as to ensure an additive content of from 0.5 to 20 wt % in the silicon nitride compact; heating the compact under a pure nitrogen gas blanket at a temperature not exceeding 1500.degree. C. to convert the silicon into reaction bonded silicon nitride; and sintering the reaction bonded silicon nitride compact by heating in a nitrogen gas atmosphere at a temperature of at least 1500.degree. C. Furthermore, it is taught that the silicon powder size is from 0.1 to 44 microns in size and of high purity or containing only very small amounts of nitriding catalysts. The Martinengo et al. patent teaches that any conventional sintering additive may be used. Best results are said to be achieved by using MgO, and especially in combination with Y.sub.2 O.sub.3. Other preferred additives mentioned in the patent are MgO, Y.sub.2 O.sub.3, CeO.sub.2, ZrO.sub.2, BeO, MgsN.sub.2, and AlN. Other examples of additives are given as Mg.sub.2 Si, MgAl.sub.2 O.sub.4, and rare earth additions such as La.sub.2 O.sub.3. Also iron can be used with advantage, usually in mixture with conventional additives such as MgO, YaO.sub.3, and CeO.sub.2.
It is, therefore, a primary object of the present invention to provide a method of enhancing the nitridation of silicon so that a more complete chemical conversion to silicon nitride may be achieved.